The present invention relates to a method for regulating MF or HF sputtering processes. Plasma sputtering is an established technology for coating substrates, in particular with ceramic or other multi-component functional layers. Sputtering, in particular reactive magnetron sputtering, is based on sputtering of metallic targets in a reactive atmosphere with the aim of allowing the desired combination of desired microstructure and phase composition to grow on the substrate at a high rate.
A substantial problem results therefrom however in that the reactive gas partial pressure in conventional sputtering processes cannot be altered continuously. The parameter range therefore breaks down into the defined ranges xe2x80x9ccompound modexe2x80x9d, with high reactive gas partial pressure and a target surface covered completely with reaction products and also with stoichiometric layers on the substrate, and with a xe2x80x9cmetallic modexe2x80x9d with a low reactive gas partial pressure in the sputtering chamber, an extensively metallic target surface and with growth of metallic layers on the substrate. Between these ranges, a continuous transition is in general not possible but rather unstable process conditions result, the materialisation of which is outlined in the following. For example, reference is made thereby to the reactive gas O2, however the mentioned mechanisms apply also for sputtering in N2, CHx and the like.
At the beginning of the magnetron sputtering, reactive gas is added into the sputtering chamber. Then competing growth and etching processes take their course on the target surface. With a low O2 partial pressure, the rate for the growth of the oxide covering is low, so that the etching process predominates by the sputtering removal of the oxide layer. The target surface remains therefore in total metallic. This state is stable since the target operates as an efficient getter pump, the effective suction capacity of which often is a multiple of the suction capacity of the turbo pump which is actually used for evacuation.
If the reactive gas partial pressure is increased, then an oxide layer grows on the target surface at a somewhat higher rate. In the case of low ion flow densities and hence a low etching rate, the growth process then predo minates. In this manner, target regions covered with reaction products are produced which are also described as xe2x80x9cpoisoned target regionsxe2x80x9d.
These poisoned target regions always have a lower sputtering yield relative to the metal so that altogether less target material is sputtered. This leads to a reduction in the material removal on the target, a lowering of the target suction capacity and hence to a further increase in the reactive gas partial pressure.
As soon as the reactive gas partial pressure exceeds a critical value, the consequence is a self-increasing effect, since the increase in the reactive gas partial pressure results in a reduction in the suction capacity of the target getter pump, as a result of which an increase in the reactive gas partial pressure results in turn. This instability characterises the transition from the metallic mode into the compound mode.
Precisely the stabilisation of the discontinuous transition between these two states is technically of great interest now since, on the one hand, the growth rate in the compound mode conditioned by the low sputtering yield is only low but, on the other hand, the layers in the compound mode grow with reactive gas excess, as a result of which unfavourable layer properties result. On the other hand, the reactive gas partial pressure is generally too low in the metallic mode so that absorbing substoichiometric compounds then grow.
The desired stoichiometric layers can however be deposited at a higher rate when the process is operated precisely in the unstable transition range which is described also as xe2x80x9ctransition modexe2x80x9d. The stabilisation of this discontinuous transition state xe2x80x9ctransition modexe2x80x9d is possible by means of control cycles which take into account the dynamic behaviour of the sources and which thus can maintain the technically interesting unstable operating points.
Various control variables are proposed in the state of the art in order to stabilise the transition mode.
It is known for example from J. Affinito, R. R. Parsons, xe2x80x9cMechanisms of voltage controlled, reactive, planar magnetron sputtering of Al in Ar/N2 and Ar/O2 atmospheresxe2x80x9d, J. Vac. Sci. Technol. A 2 (1984) 1275, for materials such as ZnO, SnO2 or SiO2 in which the plasma impedance depends significantly upon the target covering, that the plasma impedance is maintained constant as a control variable by adaptation of the discharge output of the reactive gas flow (impedance control). Alternatively, it was proposed by EP 0 795 623 A1 (publication 17/09/1997) that, in oxidic systems, in which the plasma impedance depends only slightly upon the target covering so that the impedance regulation fails as for example for TiO2, Nb2O5, the reactive gas partial pressure is used as control variable which is determined via lambda probes.
The dissertation by J. Strxc3xcmpfel, xe2x80x9cProcess stabilisation with reactive high rate sputtering by means of optical emission spectroscopy for industrial production of indium-tin oxide layers and titanium dioxide layersxe2x80x9d, Chemnitz, 1991, describes the use of the optical emission of the plasma discharge as control variable as a further possibility. A combination of the above mentioned methods from the state of the art is also usable. All the above mentioned methods are also in principle usable both in the direct voltage and in MF operation.
All the known methods from the state of the art calculate absolute values. Such a calculation of absolute values is however very problematic in production conditions. Because, for example the plasma impedance is changed by target erosion, by contamination of the measuring heads, by random layers and the like.
It is therefore the object of the present invention to indicate a method for controlling sputtering processes by means of which the transition between the different operating modes, as described above, can be reliably stabilised.
This object is achieved by the method according to claim 1 and the device according to claim 7. Advantageous developments of the method according to the invention are given in the dependent claims.
The present invention presents a new method for plasma diagnostics and characterisation of the target state via the harmonic analysis of electrical discharge parameters. Included herein is for example the analysis of the complex Fourier spectrum of electrical discharge parameters, such as cathodic current or cathodic voltage. Hence, new possibilities for process regulation are created. The harmonic oscillation components of current and voltage, the phase displacement between current and voltage at the fundamental frequency and/or at higher frequencies or also the ratios of the Fourier coefficients of current and voltage of the cathode are possible as adjustable variable.
The particular advantage of the method according to the invention resides in being able to determine the target state without there being a requirement for precisely determining absolute values.
In the following, a few examples of the method according to the invention are described.